1. Field of the Invention
The present invention relates to a drive device suitable for mounting in a working vehicle such as for example a bulldozer and a drive method thereof
2. Description of the Related Art
A conventional drive device comprises an engine, transmission and drive wheel (sprocket wheel). The transmission comprises a forward running stage that is selected by engagement of a forward clutch, a reverse running stage that is selected by engagement of a reverse clutch and speed stages that are selected by a speed-stage changeover clutch.
In this drive device, when the operator operates the running control device to output a forward running command, this forward running command causes the forward clutch to be engaged so that the motive power of the engine is transmitted to the drive wheels through the forward running stage and the currently selected speed stage of the transmission. As a result, the vehicle runs in the forward direction. Also, in this drive device, when the operator operates the running control device to output a reverse running command, this reverse running command causes the reverse clutch to be engaged so that the motive power of the engine is transmitted to the drive wheel through the reverse running stage and the currently selected speed stage of the transmission. As a result, the vehicle runs in the reverse direction.
Typically, when forward/reverse running action of the vehicle is changed over, changeover of forward/reverse running action is preferably performed by the running control device after temporarily stopping the vehicle, in order to protect the device.
However, in order to shorten the cycle time, the actual situation is that a reverse running command is output by operating the running control device for example in a condition with the vehicle running forward.
In the drive device, if the operator causes a reverse running command to be output by operating the running control device while the vehicle is running in the forward direction, the reverse clutch is gradually brought into engagement. In this way, the kinetic energy of the vehicle is absorbed by the reverse clutch and the vehicle is decelerated.
For example Japanese Patent Application Laid-open No. 6-272758 discloses a drive device wherein, in the event of changeover between forward and reverse running, control is exercised so as to decelerate the vehicle by absorbing the kinetic energy of the vehicle by means of the speed stage changeover clutch, by putting both the forward clutch and reverse clutch in disengaged condition and suitably engaging the speed stage changeover clutch.
Using an action of engaging the reverse clutch to decelerate the vehicle during forward running as in conventional drive devices is subject to the following problems.
(A) When the reverse clutch is put into a slipping action condition, motive power so as to cause the vehicle to run in the reverse direction is instantaneously transmitted from the transmission to the drive wheel. This therefore gives rise to a gear change shock.
(B) In order to decelerate the vehicle, the reverse clutch is subjected to a large heat load. The life of the reverse clutch is thereby lowered. Consideration has been given to performing the action of engaging the reverse clutch for a short time, in order to suppress the heat load to which the reverse clutch is subjected, but, if this is done, rapid deceleration of the vehicle generates a large gear change shock.
(C) In order to suppress the application of further heat load to the reverse clutch after this clutch has received a large heat load during vehicle deceleration, it is necessary to perform an action of engaging the reverse clutch for a short time when the vehicle is made to form reverse running after deceleration. Such an engagement action of course gives rise to gear change shock.
Problems similar to the above problems also arise in the case of changing over to forward running of the vehicle during reverse running.
Also, although, in the case of the drive device of Laid-open Japanese Patent Application Laid-open No. 6-272758, both the forward clutch and the reverse clutch are put in disengaged condition when changeover of the forward/reverse running is performed, so that there is no possibility of instantaneous drive force such as would advance the vehicle in the direction opposite to its current direction of running being applied from the transmission to the drive wheel, so a gear change shock as described under (A) cannot be produced, the speed stage changeover clutch is subjected to a large heat load during vehicle deceleration. As a result, there is the problem that the life of the speed stage changeover clutch is decreased.